The foundry industry uses sand, such as silica, chromite or olivine sand, extensively in forming molds which are suitable for casting molten metals. In forming the molds, the sand is combined with various binding agents. Usually, the binding agents employed are natural binders, such as linseed oil and bentonite, and chemical binders, such as organic resins. The type of the binding agents employed is dependent on the desired molding properties. However, bentonite and organic resin binders are widely utilized. Most of the organic resin binders are based on phenolic and furannic resins that form reticular structures under the influence of a catalyst together with or without the application of a moderate temperature.
The foundry industry recycles large quantities of spent sand having binder residues. Most often, the spent sand is recycled after being subject to a mechanical/attrition treatment followed by a screening step. The mechanical/attrition treatment allows to remove or screen out the binder residues that have been broken down to extremely fine particles. Such a treatment, however, also causes the sand grains to break and erode, thus resulting in removing or screening out large quantities of the sand with the binder residues. Typically around 20% of the sand is lost in such an operation. That is, millions tons of the sand are disposed worldwide annually as a waste. Even though a large quantity of the disposed spent sand contains bentonite (referred to as "green sand") and may be harmless to the environment, it is often combined or mixed with spent sand containing organic binders due the employment of bentonite and organic binders for making the different parts of a mold and/or due to the complexity of the foundry industry's operation. The disposed spent sand having the organic binders is normally hazardous to the environment.
To avoid the inefficiency and environmental hazard associated with the above recycling method, several thermal sand reclamation processes involving a fluid bed have been proposed. In these processes, electricity or natural gas is used for auxiliary heating while air is normally used as a fluidizing medium and as a means for burning organic residues present on the sand. These processes are useful for continuously treating large quantities of sand containing substantially identical binders and having substantially identical granulometry. However, they are neither effective nor efficient in treating different sands, i.e., sands having different granulometry and different binders, sequentially or in mixture since different operating parameters are needed for different sands. Moreover, crushing the spent (used) sand clods to very fine mesh for the fluidized bed treatment is a process handicap.
Consequently, WO 91/08068 has proposed a different thermal process for roasting foundry sand. Initially, the contaminated spent sand is charged into a rotatable furnace. The furnace rotates about an axis at an angle ranging from about 5.degree. to about 15.degree. , measured from vertical. Oxygen is injected at the bottom of the sand batch and diffuses throughout the sand batch. In the meantime, a flame front provided from a burner on the top of the furnace is directed to a upper surface of the sand batch. After the flame from the burner is ceased, oxygen is continuously injected to cause a progressive descent of the flame front until complete combustion of the contaminants has taken place. This thermal process, however, may suffer from certain disadvantages. First, the flame front may not descent progressively toward the bottom of the sand batch, when the sand contains limited burnable contaminants. The flame front from the top may be able to combust contaminants on the upper layer of the sand batch, but may not be able to reach the bottom layer of the sand batch. Second, the desired temperature uniformity may not be obtained since the flame from the burner, i.e., the tip of a flame, contacts only a small area of the upper layer of the sand batch. A certain portion of the sand batch, especially those at the bottom, may not be subject to the flame front and may still have contaminants when the operation is ceased. Third, the sand grain may be fractioning due to thermal shock since the sand grain is subject to rapid heating as the fire front progresses downward. The body of the sand batch, for example, may be subject to thermal shock because it does not appear to be preheated. Finally, an off gas containing substantial amounts of the partially pyrolized organic contaminants and CO may be released to the atmosphere since the injection rate for oxygen diffusing through the layer of the spent sand batch is normally kept at a pretty low level to avoid, among other things, channelling and local fluidization of the sand batch.
It is an object of the invention to reduce or eliminate the presence of CO and partially pyrolized hazardous organic matter in the off gas exiting a foundry sand roasting rotary furnace.
It is another object of the invention to promote temperature uniformity within a rotary furnace, i.e., the head space and sand batch within a rotary furnace, during pyrolysis.
It is yet another object of the invention to provide ways to control the temperature within a rotary furnace during pyrolysis and combustion to minimize any alteration of the sand grain structure.
It is a further object of the invention to reduce dust entrainment in the off gas exiting a rotary furnace.
It is an additional object of the invention to provide a thermal process useful for treating different sands effectively and efficiently.
It is an additional object of the invention to provide a thermal process useful for treating and decontaminating spent sand that has to be disposed of, such as sand fines and dust, so that such a disposal is harmless to the environment.
It is an additional object of the invention to allow the use of an iron melting rotary kiln for pyrolyzing spent sand and combusting the resulting gas during dwell times.